Method of preparing composite castings



United States Patent 3,279,006 METHOD OF PREPARING COlVIPOSITE CASTINGSCharles W. Schwartz, Whitehall, Mich., and Harold L. Wheaten, RollingMeadows, 11]., assignors to lHartin Metals Company, a corporation ofDelaware No Drawing. Filed Dec. 30, 1963, Ser. No. 334,971 12 Claims.(Cl. 22-204) This application is a continuation-in-part of our copendingapplication Serial No. 191,415, filed May 1, 1962.

The present invention relates to the casting of metals and alloys. Moreparticularly, it relates to composite metallic objects and to a methodfor metallurgically bonding metallic materials of dissimilarcompositions and/or properties.

Briefly, the invention comprises a method of producing composite objectswherein molten metallic material is poured into contact with and bondedto a solid metallic object positioned within a refractory mold beingmaintained under a protective atmosphere, such as a high vacuum, whichmold, prior to the pouring of the metallic material, has been treatedunder vacuum at an elevated temperature in the range between about 1000F. and the temperature of incipient melting of the solid metallic objector the slump temperature of the refractory mold, whichever is thelimiting upper temperature, and at the time of pouring, the molten metalis at a temperature between about 200 F. and 2000 F. higher than themold temperature.

Composite metallic objects prepared in accordance with the method ofthis invention contain a cast metallic portion, a metallic object ofsuitable configuration which was solid at the time of pouring the castportion and a bonding layer formed between said object and said castportion by the interalloying of the metallic object with the pouredmetal to produce a metallurgically bonded zone.

Metals and alloys are selected for use as structural materials generallyon the basis of qualifications adapting them to several requirements,i.e., high strength per unit weight, high temperature strength,resistance to oxidation, etc. The structural materials may exhibit oneor more of the characteristics but generally are deficient in somerespect which drastically limits the conditions of safe operating use.The deficiencies of various metals and alloys may be illustrated byreference to metals characterized by a high melting point. Therefractory metal tungsten, for example, has the highest melting point ofany metal (6170 F.), but its use for components subject .to hightemperature conditions, is limited to atmospheres free of oxygen becauseat temperatures above 1800 F. in air, tungsten oxidizescatastrophically. Oxidation is also a serious problem with other membersof the metals characterized by high melting points such as molybdenum,vanadium, columbium, etc.

Attempts have been made to overcome the deficiencies of structuralmembers prepared from metals and alloys by, for example, changing thecomposition of the surface of a member, by providing a new surface layerthrough welding or brazing of a metal sheet to a base, forming a layerabout the base, and the like. Another method for providing a 'newsurface layer on a metallic base is to coat a base with a metal such aszinc by dipping, such dipped coating having the ability to readilyadhere to the base, and pouring a metallic surface layer underconditions to bond the poured metal to the adhered metal. A change inthe surface of the base may be accomplished, for example, by carburizinga surface layer, by depositing on and diffusing into the surface of abase metal a protective coating such as chromium, aluminum, and thelike, as by the so-called vapor deposition technique. Base metals havingsurfaces altered by the means heretofore in use, have not beencompletely acceptable where structural parts were subject to hightemperature environments. For example, a vapor deposited surface coatingdoes not maintain a constant composition but is subject to alteration bydiffusion of elements into the surface layer. A thin diffused surfacelayer tends to disappear through a continuance of the diffusion processat high temperature which creates a new exposed alloy as distinct from,for example, a metallic base covered by an insulating layer of aspecific metal or alloy.

Now, it has been discovered that a composite object can be formed withpoured metallic material joined at one or more surfaces to a metallicobject or objects and in thickness varying from a thin surface layer ofsufiicient thickness to be an effective protective barrier around ametallic core to a structural member active as a link between solidmetallic objects or an appendage to a solid metallic object, with aproper bonding or transition layer of intermediate compositional makeup.

By protective barrier is meant a continuous surface layer, uniformlybonded to a core and being of cont-rollable thickness, which layers arecommonly called claddings.

A composite metallic object may be produced by melting a metallicmaterial having a specific property desired in the poured portionthereof under vacuum, heating a refractory mold having a cavity thereinadapted to receive melted metallic material and having a solid metallicobject or element positioned therein with at least a portion of thesurface thereof exposed within the cavity, under vacuum and pouring themolten metallic material while maintaining an inert atmosphere. Thetemperature to which the mold is heated will vary with such factors asthe nature of metals and/ or alloys being bonded, and configuration ofthe solid object within the mold and the area of the bonding zone, therelative masses of the solid and poured portions of the composite objectto be formed, and the like. When dealing with so-called refractory metalalloys which are sensitive to the presence of small quantities of gases,the mold is preferably heated to a temperature at or above the pouringtemperature of the molten material but not in excess of the slumptemperature of the mold or the temperature of incipient melting of .thesolid metallic object, whichever is the limiting upper temperature. Theheating of the refractory mold under vacuum is carried out for a periodof time sufficient to permit the desired degree of evacuation ofvolatiles, for example, for a period from 5 minutes to minutes.Following the vacuum treatment, the mold is cooled, if necessary, to atemperature establishing a satisfactory solidification gradient whilemaintaining a nonoxidizing atmosphere or vacuum, and the molten materialis poured into the mold and the casting is cooled in vacuum ornon-oxidizing atomsphere until solidified.

In accordance with this invention, a suitable mold capable ofwithstanding the heat treatment without slumping is treated under theinterrelated conditions of temperature and vacuum, to accomplish apredetermined degree of degasification. For example, the mold may betreated at a relatively low temperature for an extended period or at ahigher temperature for a shorter period for removal of a portion of thevolatiles. However, when such conditions are encountered assusceptibility of the solid object to oxidation or the sensitivity of aso-called refractory alloy to small amounts of gas in a mold, it ispreferable to utilize vacuum and temperature conditions which willrender the mold substantially inert at the temperature and vacuumconditions which will affect it when the molten metal is poured into themold cavity. It will be recognized that inertness of the mold is notnecessary to obtain sound metallurgically bonded zones with some alloysas some are not as sensitive to the presence of volatiles as otheralloys.

The mold may have a metallic object or objects positioned therein at thetime of forming the mold or inserted therein at a time prior to pouringof the molten material.

When the conditions dictate that the mold should be rendered inert bythe removal of gas to the extent that substantially no volatiles will beexpelled from the mold when the metal is being poured into the cavitythereof, quick conditioning of the mold can be attained by heating undervacuum to a high temperature, preferably close to an upper limitingtemperature, i.e., the temperature of incipient melting of the solidmetallic object or the slump temperature of the mold, and be heated atthat temperature for a time sufficient to permit effective degassing Inorder to achieve effective degassing, the entire mold and the metallicobject within the mold must be brought to the desired temperature.Inasmuch as heat transfer through the relatively dry mass of solidparticles of the mold is a slow process, the time of heating varies withthe thickness of the mold. If the melting point of the solid metallicobject is higher than the softening temperature of the refractory mold,the temperature to which the refractory mold may be raised in apreconditioning operation may be varied with the melting point and morespecifically the pouring temperature of the molten metal, which pouringtemperature is usually between 50 F. and 600 F. above the melting pointtemperature for the material being rendered molten. A preferredprocedure, particularly when preparing claddings, is to heat the mold toa temperature above the pouring temperature of the molten metal foreffective degassing and then to cool the mold to between 200 F. and 1200F. below the pouring temperature of the molten metal to provide asuitable solidification gradient as is explained herein later. In thepouring of metal castings designed to operate at high temperatures, forexample, blades, vanes or discs of gas turbine engines, the alloygenerally melt at a temperature in the range between about 2400" F. and2700 F. and are poured at temperatures in the range between about 2450F. and 3000 E, into a refractory mold whose temperature is in the rangebetween about 1600 F. and 2400 F.

One of the primary requisites for the successful operation of thisprocess is a clean surface on the metallic object. The metallic objectmust be free, for example, of an oxide layer at the time of contact withmolten metal if a sound metallurgical bond is to be formed. Generally,the heating of the mold and the meallic object under vacuum prevents orminimizes formation of oxide on the object during the steps just priorto casting. If, during the processing, for example, during the waxburnout, a metallic object cannot be prevented from acquiring an oxidelayer, the oxide formation should be removed by means such as thermal orchemical treatment prior to pouring the molten metallic material.Alternatively, sectional shell molds, which permit insertion of metallicobjects after dewaxing, can be used. Such sectional moldspermit'introduction of a metallic object after the dewaxing and theburn-out under oxidizing conditions which could result in the formationof an oxide coating on the metallic object.

Also, in the preparation of composite metallic objects, it is desirablethat the refractory mold be substantia'lly free from contaminants.Vacuum treatment of the mold at relatively high temperatures willeliminate contaminants and in addition may serve the purpose of removinga volatile oxide layer from a solid metallic.

object, such as molybdenum, tungsten, vanadium, etc..

If a refractory mold having a solid metallic object.

positioned therein, such as a metallic core, has been subjected to apreheat treatment at a high temperature, for:

examples, below the temperature of incipient melting of the solidmetallic object, but above the pouring temperature of the moltenmaterial, the mold is cooled prior to pouring of molten metal or alloyto a temperature sufliciently lower than the pouring temperature of themolten metal to establish a temperature difference such that asatisfactory solidification gradient can be developed. A pretreatedrefractory mold is generally cooled to a temperature between 200 F. and2000 F. below the pouring temperature of the molten metal to establish athe choice of the temperature to which the mold and enclosed solidobject will be cooled prior to pouring molten metal, assuming that thepouring temperature of the molten mass would be the same and theconfiguration of the poured portion of the composite object also wouldbe similar, is the relative masses of the solid and poured portions ofthe composite object. the solid object is large relative to the mass ofthe poured portion of the composite object, one way to control theextent of the bonding zone is to utilize a relatively hot mold so thatthe molten metal will not be chilled before it has ample opportunity tobond to the solid object. If the mass of the solid object is smallrelative to the mass of the poured portion of the composite object, oneway to control the depth of the bonding zone developed, is to utilize amold having a relatively low temperature within the bounds described,and thereby speed initiation of solidification and cooling of the entiremass of metal Within the mold to temperatures below those conducive tobe utilized at the time of pouring so that a particular combination mayrequire utilization of a differential in temperature between the moldand poured metalwhich are at variance with the differentials indicatedin the above discussed specific instances. The objective of control ofsolidification is usually attained by cooling the mold to a temperaturein the-range between about 1000 F. and 2400 F., and preferably in therange between 1600 F. -and'2400 R, if the pouring temperature of themetal is between about 2450 F. and 3000 F. and the solid metallic objectis of a type permitting high temperature pretreatment of the mold.Castings poured when. the mold is at an appropriate temperature toestablish.

a solidification gradient exhibit a proper fill-out and adequate feedingso that the defects known as porosity, shrinkage, stress cracks due todifferential expansion characteristics, etc., will be minimized oreliminated.

Molten metal is poured into a pretreated mold while maintaining aprotective atmosphere. After solidification, cooling of the casting andmold can be completed under atmospheric conditions, following which thecasting is processed in the conventional manner of knockout, cutoff andfinish grinding. As used in the claims, the language protectiveatmosphere is intended to include holding under vacuum, in an atmosphereof inert gas or in an atmosphere of non-oxidizing gas.

If the mass of In a preferred embodiment of this invention, a moldsuitable for use in the casting process of this invention is made from amaterial which is substantially non-reactive at temperatures above themelting point of the metal to be cast. Highly refractory, relativelyinert materials such as quartz, zircon, zirconia, alumina, and the like,have been found to meet these requirements.

A mold formed from suitable non-reactive materials and produced withsuitable sprues, gates, and the like, is preferably subjected totreatment in a chamber maintained at a vacuum of less than 100 microns,usually of less than 10 microns and if a substantially inert mold isdesired, in a chamber maintained at a vacuum in the range between about0.01 and microns.

In preparing a shell or a refractory mold of a type useful in a processof this invention, models of the components 'of the casting are preparedfrom expendable pattern materials such as wax and equivalents thereof.The metallic object or a wax facsimile thereof and the other componentsare joined together in the desired form and a refractory mold formed bythe conventional dip and stucco procedure. Refractory materials utilizedin the dip and stucco operations are those which are substantiallynonreactive at temperatures above the pouring temperature of the moltenmetal to :be cast.

The refractory mold is heated to melt out the wax and to produce thepouring cavity. If the metallic object is not in the shell, the dewaxingmay be accomplished by firing the mold at a temperature between 1300 F.and 1850 F. to remove any residual wax or other unwanted material suchas carbonaceous or organic substances. If the metallic object isintegral with the mold, the dewaxing may be accomplished by relativelylow temperature thermal means or by chemical means such as by use ofsolvents which do not produce an oxide layer on the metallic object.Suitable solvents for the purpose are trichloroethylene, carbontetrachloride, and the like.

One form of apparatus for carrying out the casting of composite metallicobjects consists of a vacuum chamber having a melting furnace therein, apumping system communicating with the interior of the chamber adapted toevacuate gases from the chamber and a heater station adapted to receivea mold moved into the vacuum chamber through a mold charging lockprovided with apparatus for positioning the refractory mold in thepouring station within the heater.

The process for the preparation of the composite metallic objects ofthis invention permits wide choice in the properties and characteristicsof both the solid metallic object and the cast metal or alloy.

The solid metallic object may vary from a single metal such asmolybdenum, tungsten, columbium, tantalum, chromium, vanadium, hafnium,zirconium, titanium, uranium, iron, cobalt, nickel and alloys thereof.Terms used throughout this specification, such as molybdenum, tungsten,etc., are intended to cover the individual metal and alloys thereof.Likewise, the poured metal may vary from a single metal such asaluminum, nickel, etc., to alloys of a complex nature such as cobaltbaseand nickel-base alloys. Combinations of interest are metallic cores cladwith alloys having oxidation resistance at high temperatures, as well asother embodiments, such as wear resistant clad layers over tough,ductile core material or clad ingots which can subsequently be rolled.In still another embodiment, a solid metallic object may, by appropriatearrangement, be positioned to come in contact with molten metal onlyalong a single surface or plane so that the molten metal solidifies toform an appendage of the composite object extending from said surface.

As used in the claims, the langage the temperature of incipient meltingof the metallic object but below the slump temperature of the mold isintended to mean that the refractory mold is heated to some elevatedtempera- Example I A tungsten core is built into a wax replica of thedesired casting shape together with wax replicas of gates, runner bars,etc. The wax replica of the overall pattern is invested with a suitablezircon mold by the dip and stucco procedure. After room temperaturedrying, the coated pattern is fired and dewaxed by a 10 secondflashheating to about 1800 F. followed by low temperature dewaxing at atemperature of about 200 F. The heating operation removes most of thewax; the remainder of the wax adhering to the interior surface of themold is removed by washing with trichloroethylene solvent.

This dewaxed mold is introduced into the vacuum chamber through a moldlock into position within the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2800 F. and held at approximately that temperature for30 minutes while a vacuum of less than 10 microns, as measured by aPhillips gauge, is maintained. After holding the mold for theabove-stated time, the mold is cooled to approximately 1900 F.

Metallic elements for the cladding layer are introduced into aninduction melting furnace operating within the evacuable chamber inquantities to form an alloy having the following composition:

13% chromium, 4.5% molybdenum, 2.25% columbium, 6.0% aluminum, 0.6%titanium, 0.01% boron, 0.08% zirconium, 0.15% carbon, and the balancenickel.

The metallic elements are reduced to molten form by heating toapproximately 2500 F.

After the mold has been cooled to about 1900 F., the molten alloy ispoured into the mold. When the pouring operation is complete, thecomposite object is held in the vacuum chamber for about 30 minutes,following which the mold is removed from the chamber through the moldlock and cooled to room temperature. The composite object having anickel-base alloy, as an approximately inch thick surface layer, clad toa tungsten core, has a smooth surface substantially free of pits andporosity. A study of the internal structure showed a metallurgicallybonded layer substantially free of defects.

Example II A molybdenum core is built into a wax replica of the desiredcasting shape together with wax replicas of gates, runner bars, etc. Thewax replica of the overall pattern is invested with a suitable zirconmold by the dip and stucco procedure. After room temperature drying, thecoated pattern is fired and dewaxed by a 10 second flash heating toabout 1800 F. followed by low temperature dewaxing at a temperature ofabout 200 F. The heating operation removes most of the wax, theremainder of the wax adhering to the interior surface ofvthe mold isremoved by washing with trichloroethylene solvent.

This dewaxed mold is introduced into the vacuum chamber through a moldlock into position Within the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2900 F. and held at approximately that temperature for25 minutes while a vacuum of less than 10 microns, as measured by aPhillips gauge, is maintained. After holding the mold for theabovestated time, the mold is cooled to approximately 1800" .F.

Metallic elements for the cladding layer are introduced into aninduction melting furnace operating within the evacuable chamber inquantities to form an alloy having the following composition:

22% chromium, 9% molybdenum, 0.6% tungsten, 18.5% iron, 2% cobalt, 0.10%carbon, and the balance nickel.

The metallic elements are reduced to molten form by heating toapproximately 2500 F.

After the mold has been cooled to about 1800" F., the molten alloy ispoured into the mold. When the pouring operation is complete, thecomposite object is held in the vacuum chamber for about 30minutes,.following which the mold is removed from the chamber throughthe mold lock and cooled to room temperature. The composite objecthaving a nickel-base alloy, as an approximately 0.1 inch thick surfacelayer, clad to a molybdenum core, has a smooth surface substantiallyfree of pits and porosity. A study of the internal structure showed ametallurgically bonded layer substantially free of defects.

Example III A pre-formed core has the following composition:

21 /2% chromium, 10% tungsten, 9% tantalum, 0.2% zirconium, 0.85%carbon, 0.01% boron, 1% iron, and the balance cobalt.

The alloy core is inserted into a wax replica of the desired castingshape together with wax replicas of gates, runner bars, etc. The waxreplica of the overall patterns is invested with a suitable zircon moldby the dip and stucco procedure. After room temperature drying, thecoated pattern is fired and dewaxed by a 10 second flash heating toabout 1800" F. followed by low temperature dewaxing at a temperature ofabout 200 F. The heating operation removes most of the wax, theremainder of the wax adhering to the interior surface of the mold isremoved by washing with trichloroethylene solvent.

This dewaxed mold is introduced into the vacuum chamber through a moldlock into position within the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2200 F. and held at approximately that temperature for50 minutes while a vacuum of less than 10 microns, as measured by aPhillips gauge,

vis maintained. After holding the mold for the abovestated time, themold is cooled to approximately 1750 F.

Metallic elements for the cladding layer are introduced into aninduction melting furnace operating within the evacuable chamber inquantities to form an alloy having the following composition:

22% chromium, 9% molybdenum, 0.6% tungsten, 18.5% iron, 2% cobalt, 0.01%carbon, and the balance nickel.

The metallic elements are reduced to molten form by heating toapproximately 255 F.

After the mold has been cooled to about 1750 F., the molten alloy ispoured into the mold. When the pouring operation is complete, thecomposite object is held in the vacuum chamber for about 45 minutes,following which the mold is removed from the chamber through the moldlock and cooled to room temperature. The composite object having anickel-base alloy, as an approximately 0.05 inch thick surface layer,clad to the alloy core, has a smooth surface substantially free of pitsand porosity. A study of the internal structure showed a metallurgicallybonded layer substantially free of defects.

Example IV A columbium core is built into a wax replica of the desiredcasting shape together with wax replicas of gates, runner bars, etc. Thewax replica of .the overall pattern is invested with a suitable zirconmold by the dip and stucco procedure. After room temperature drying, thecoated pattern is fired and dewaxed by a second flash heating to about1800 F. followed by low temperature dewaxing at a temperature of about200 F. The heating operation removes most of the wax, the remainder ofthe wax adhering to the interior surface of the mold is removed bywashing with trichloroethylene solvent.

This dewaxed mold is introduced into the vacuum chamber through a moldlock into position within the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2750 F. and held at approximately that temperature for30 minutes while a vacuum of less than 2 microns, as measured by aPhillips gauge, is maintained. After holding the mold for theabovestated time, the mold is cooled to approximately 1850 F.

Metallic elements for the cladding layer are introduced into aninduction melting furnace operating within the evacuable chamber inquantities to form an alloy having the following composition:

20% chromium, 0.12% carbon, 0.4% titanium, and the balance nickel.

The metallic elements are reduced to molten form by heating toapproximately 2450 F.

After the mold has been cooled to about 1850 F., the

molten alloy is poured into the mold. When the pouring operation iscomplete, the composite object is held in the vacuum chamber for about30 minutes, following which the mold is removed from the chamber throughthe mold lock and cooled to room temperature. The composite objecthaving a nickel-base alloy, as an approximately 0.08 inch thick surfacelayer, clad to a columbium core, has a smooth surface substantially freeof pits and porosity. A study of the internal structure showed ametallurgically bonded layer substantially free of defects.

Example V A wax replica of a turbine blade is joined with a riser. andpouring basin assembly adapted to provide a split mold separating at theparting line of the cavity, and the wax replica of the overall patternis invested with a suitable zircon mold by the dip and stucco procedureusing zircon and alumina. After room temperature drying, the coatedpattern is fired and dewaxed by heating in an oven to about 1800 F. forabout 30 minutes.

After cooling the shell to room temperature, an air foil section of aturbine blade is inserted into the split mold which leaves a cavity. forthe pouring of the root (bottom) section and the shell sealed withrefractory slip to prevent leaking. This air foil section has thefollowing composition:

0.15% carbon, 9% chromium, 10% cobalt, 12.5% tungsten, 1% columbium, 2%titanium, 5% aluminum, 0.05% zirconium, 0.02% boron, and the balancenickel.

This sealed shell is introduced into the vacuum chamber through a moldlock into position within the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2000 F. and held at this temperature for 30 minuteswhile a vacuum of less than 10 microns is maintained.

Metallic elements for the appendage are introduced into an inductionmelting furnace operating within the evacuable chamber in quantities toform an alloy having the following composition: i

0.15% carbon, 11% chromium, 20% cobalt, 5% molybdenum, 1.5% titanium, 5%aluminum, 0.05% zirconium, 0.015% boron, and the balance nickel.

The metallic elements are reduced to molten form by heating toapproximately 2700 F.

The molten alloy at approximately 2700 F. is poured into the mold heldat 2000 F. When the pouring operation is complete, the composite turbineblade is held in the vacuum chamber for about 30 minutes, followingwhich the mold is removed from the chamber and cooled to roomtemperature. The composite turbine blades show a metallurgically bondedzone.

Example VI A replica of a turbine blade is joined with a riser andpouring basin assembly adapted to provide a split mold separating at theparting line of the cavity and the wax replica of the overall pattern isinvested with a suitable zircon mold by the dip and stucco procedureusing zircon and alumina. After room temperature drying, the coatedpattern is fired and dewaxed by heating in an oven to about 1800 F. forabout 30 minutes.

After cooling the shell to room temperature, an air foil section of aturbine blade and a forged root section of a turbine blade are insertedinto the split mold so as to leave a short transition zone into whichmolten alloy is to be poured and the shell sealed with refractory slipto prevent leaking.

The air foil section has the following composition:

0.15% carbon, 9% chromium, 10% cobalt, 12.5% tungsten, 1% columbium, 2%titanium, 5% aluminum, 0.05% zirconium, 0.02% boron, and the balancenickel.

The forged root section has the following composition:

0.06% carbon, 15% chromium, 19% cobalt, 5.15% molybdenum, 3.5% titanium,4.4% aluminum, 0.05% zirconium, 0.03% boron, 0.4% iron, and the balancenickel.

This sealed shell is introduced into the vacuum chamber through a moldlock into position within .the mold heater which consists of a graphiteinduction susceptor and a surrounding induction coil. The mold is heatedto approximately 2000 F; and held at this temperature for 30 minuteswhile a vacuum of less than microns is maintained.

Metallic elements for the transition zone are introduced into aninduction melting furnace operating within the evacuable chamber inquantities to form an alloy having the following composition:

0.15% carbon, 9% chromium, 10% cobalt, 12.5% tungsten, 1% columbium, 2%titanium, 5% aluminum, 0.05 zirconium, 0.02% boron, and the balancenickel.

The metallic elements are reduced to molten form by heating toapproximately 2700 F.

The molten alloy is poured into the mold held at 2000 F. When thepouring operation is complete, the composite turbine blade is held inthe vacuum chamber for about 30 minutes, following which the mold isremoved from the chamber and cooled to room temperature. This compositeturbine blade combines an air foil section possessing high temperaturestrength with a prefabricated root section of high strength and greaterductility at lower temperatures encountered in this section of turbineblades during engine operation, a combination of properties unattainablewith any single alloy, and a transition zone of strength and ductilityeliminating failures in this area.

In general, it will be understood that the details herein described areintended to be merely illustrative in character, and that the processmay be modified readily by those skilled in the art without departurefrom the spirit and scope of the invention as expressed in the appendedclaims.

We claim:

1. A method of forming composite metallic objects comprising heating arefractory mold having a cavity therein and having a solid metallicelement positioned therein which fills only a portion of said cavity, toa temperature in the range between about 1000 F. and the temperature ofincipient melting of the metallic element but below the slumptemperature of the mold While the mold is subjected to a vacuum of lessthan 100 microns, establishing a temperature level for the treated moldat which there is a temperature differential in the range between about200 F. and 2000 F. between the temperature of the treated mold and thetemperature of metal to be poured, said temperature diflerential beingof a magnitude effective to prevent excessive diffusion of an elementbeyond a bonding layer, pouring the molten metal into the treated moldto contact and to bond to said metallic element while the metal and themold are maintained under vacuum and maintaining vacuum on said molduntil the cast metal is solidified.

2. A method of forming composite metallic objects comprising heating arefractory mold having a cavity therein and having a solid metallicelement positioned therein which fills only a portion of said cavity, toa temperature in the range between about 1000 F. and the temperature ofincipient melting of the metallic element but below the slumptemperature of the mold while the mold is subjected to a vacuum in therange between about 0.01 and 10 microns for a period in the rangebetween about 5 minutes and about 180 minutes, cooling the treated moldwhile the vacuum is maintained thereon to a temperature at which thereis a temperature differential in the range between about 200 F. and 2000F. between the cooled treated mold and the metal to be poured, saidtemperature differential being of a magnitude effective to preventexcessive diffusion of an element beyond a bonding layer, pouring metalmelted under vacuum into the treated mold to contact and to bond to saidmetallic element while the melt and the mold are maintained under vacuumand maintaining vacuum on said mold until the cast metal is solidified.

3. A method of forming composite metallic objects comprising heating arefractory mold having a cavity therein and having a solid metallicelement positioned therein which fills only a portion of said cavity, toa temperature in the range between about 1000 F. and the temperature ofincipient melting of the metallic element but below the slumptemperature of the mold while the mold is subjected to a vacuum of lessthan microns, establishing a temperature level for the treating mold,heating molten metal under vacuum to a pouring temperature such thatthere is a temperature differential in the range between about 200 F.and 2000 F. between the temperature of the treated mold and thetemperature of metal to be poured, said temperature differential beingof a magnitude effective to prevent excessive diffusion of an elementbeyond a bonding layer, pouring the molten metal into the treated moldto contact and to bond to said metallic element while the metal and themold are maintained under vacuum and maintaining vacuum on said molduntil the cast metal is solidified.

4. A method of forming composite metallic objects comprising heating arefractory mold having a cavity therein and having a solid metallicelement melting at a temperature above about 2400 F. positioned thereinwhich fills only a portion of said cavity, to a temperature in the rangebetween 1000 F. and the temperature of incipient melting of the metallicelement but below the slump temperature of the mold while the mold issubjected to a vacuum in the range between about .01 and 100 microns fora period in the range between about 5 minutes and about 180 minutes,cooling the treated mold while vacuum is maintained thereon to atemperature at which there is a temperature differential in the rangebetween 200 F. and 2000 F. between the temperature of the treated moldand the metal to be poured, said temperature diflerential being of amagnitude eifective to prevent excessive diffusion of an element beyonda bonding layer, pouring metal melted under vacuum at a temperature inthe range between 2450 F. and 3000 F. into core melting at a temperatureabove 3000 F positioned therein under vacuum to a temperature in therange between about 1000 F. and the temperature of incipient melting ofthe metallic core but below the slump temperature of the mold while themold is subjected to a vacuum of less than 100 microns, establishing atemperature level for the treated mold at which there is a temperaturedifferential in the range between about 200 F. and 2000 F. between thetemperature of the treated mold and the temperature of metal to bepoured, said temperature differential being of a magnitude effective toprevent excessive diffusion of an element beyond a bonding layer,heating metal under vacuum to a molten state, pouring the molten metalinto the treated mold to contact and to bond to said metallic elementwhile the metal and the mold are maintained under vacuum and maintainingvacuum on said mold until the cast metal is solidified.

6. The method of forming a composite metallic turbine blade comprisingproducing a refractory mold having a cavity therein in the configurationof a turbine blade, positioning a solid air foil section in thecorresponding portion of the mold cavity, heating the mold and air foilsection under vacuum to a temperature in the range between about 1000 F.and the temperature of incipient melting of the metallic element butbelow the slump temperature of the mold while the mold is subjected to avacuum of less than 100 microns, establishing a temperature level forthe treated mold at which there is attemperature differential in therange between about 200 F. and 2000 F. between the temperature of thetreated mold and the temperature of metal to be poured, said temperaturedifferential being of a magnitude effective to prevent excessivediffusion of an element beyond a bonding layer, heating molten metalunder vacuum, pouring the molten metal into the treated mold to contactand to bond to said metallic element while the metal and the mold aremaintained under vacuum and maintaining vacuum on said mold until thecast metal is solidified.

7. The method of forming a composite metallic turbine blade comprisingproducing a refractory mold having a cavity therein in the configurationof a turbine blade, positioning a solid air foil section in thecorresponding portion of the mold cavity, heating the mold and airfoilsection under vacuum to a temperature in the range between about 1600 F.and 2400 F. while the mold is subjected to a vacuum of less than 100microns, establishing a temperature level for the treating mold at whichthere is a temperature differential in the range between about 200 F.and 2000 F. between the temperature of the treated mold andthetemperatures of metal to be poured, said temperature differentialbeing of a magnitude effective to prevent excessive diffusion of anelement beyond a bonding layer, heating molten metal under vacuum,pouring the molten metal into the treated mold to contact. and to bondto said air foil section while the metal and the mold are maintainedunder vacuum and maintaining vacuum on said mold until the cast metal issolidified.

8. The method of forming a composite metallic object comprisingproducing a refractory mold having a cavity therein of desired form,positioning a solid metallic element therein to fill one portion of saidcavity, heating the mold under vacuum to a temperature in the rangebetween about 1000" F. and the temperature of incipient melting of themetallic object section but below the slump temperature of the mold,establishing a temperature level for the treated mold at which there isa temperature differential in the range between about 200 F. and 2000 F.between the temperature of the treated mold and the temperature of metalto be poured, said temperature differential being of a magnitudeeffective ,to prevent excessive diffusion of an element beyond a bondinglayer, heating metal to molten state under vacuum, pouring the moltenmetal into the treated mold to .contact and to bond to said metallicelement as an appendage while the metal and the mold are maintainedunder vacuum and maintaining vacuum on said mold untl the cast metal issolidified.

9. The method of forming a composite metallic object.

comprising producing a refractory mold having a cavity therein ofdesired form, positioning a solid metallic object consisting of a hightemperature 'high strength alloy selected from the group of cobalt,nickel and iron-base alloys to fill-one portion of said cavity, heatingthe mold under vacuum to a temperature in the range between about 1000"F. and the temperature of incipient melting of the metallic objectsection but below the slump temperature of the mold, establishing atemperature level for the treated mold at which there is a temperaturedifferential in the range between about 200 F. and 2000" -F. between thetemperature of the treated mold and the temperature of metal to bepoured, said temperature differential being of a magnitude effective toprevent excessive diffusion of an element beyond a bonding layer,heating metal to molten state under vacuum, pouring the molten metalinto the treated mold to con-tact and to bond to said metallic elementwhile the metal and the mold are maintained under vacuum and maintainingvacuum on said mold until the cast metal is solidified.

10. The method of forming a composite metallic ture bine bladecomprising producing a refractory mold having a cavity therein in theconfiguration of a turbine blade,

positioning a solid metallic air foil section and a forged metallic rootsection in the corresponding portions of the mold cavity in a spacedrelationship, heating the mold under vacuum to a temperature in therange between about 1000 F. and the temperature of incipient melting ofthe solid metallic sections but below the slump temperature of the mold,establishing a temperature level for the treated mold at which there isa temperature differential in the range between about 200 F. and 2000 F.between the temperature of the treated mold and the temperature of metalto be poured, said temperature differential being of a magnitudeeffective to prevent excessive diffusion of an element beyond a bondinglayer, heating metal to molten state under vacuum, pouring the moltenmetal into the treated mold to contact and to bond to said metallicsections while the metal and the mold are maintained under vacuum andmaintaining vacuum on said mold until the cast metal is solidified.

11. The method of forming a composite metallic object comprisingproducing a refractory mold having a cavity therein of desired form,positioning solid metallic objects of different composition andcharacteristics in the ends of said cavity of correspondingconfiguration in a spaced relationship, heating the mold under vacuum toa temperature in the range between about 1000 F. and the temperature ofincipient melting of the solid metallic object but below the slumptemperature of the mold, establishing a temperature level for thetreated mold at which there is a temperature differential in the rangebetween about 200 F. and 2000 F. between the temperature of the treatedmold and the temperature of metal to be poured, said temperaturedifferential being of a magnitude effective to prevent excessivediffusion of an element beyond a bonding layer, heating metal to moltenstate under vacuum, pouring the molten metal into the treated mold tocontact and to bond to said metallic element while the metal and themold are maintained under vacuum and maintaining vacuum on said molduntil the cast metal is solidified.

12. A method of forming composite metallic objects comprising heating arefractory mold having a cavity therein and having a metallic corepositioned therein to a temperature in the range between 1000" F. andthe temperature of incipient melting of the metallic core but below theslump temperature of the mold while the mold is subject to a vacuum ofless than microns, establishing a temperature level for the treated moldat which there is a temperature differential in the range between about200 F. and 2000 F. between the temperature of the treated mold and thetemperature of metal to be poured, said tem. perature differential beingof a magnitude effective to prevent excessive diffusion of an elementbeyond a bond- 13 ing layer, pouring metal melted under vacuum into thetreated mold to contact and to bond to said metallic core while themetal and the mold are maintained under vacuum and maintaining vacuum onsaid mold until the cast metal is solidified.

References Cited by the Examiner UNITED STATES PATENTS 1,162,339 11/1915 Coolidge 22204 1,162,340 11/1915 Coolidge 22204 1,534,928 4/ 1925Dugan 22-2()3 1,609,747 12/ 1926 Walter 22204 1,619,835 3/1927 Summers29196.6 1,968,069 7/ 1934 Chandler 22204 2,156,998 5/ 1939 McCullough22204 2,3 09,288 l/ 1943 Young 22204 2,709,842 6/1955 Findlay 22-'57.22,806,271 2/ 1957 Operhall 26'6-34 2,817,141 12/ 1957 Toulmin 29196.6

Hathaway 75-84 Ellis et al. v 22204 X Hohn et a1 10613-8.23

I-lughes 2273 Meissner 22204 Brennan 22209 X Jominy 22204 Murphy et a1222l6.5 C-arreker et a1.

Pagonis 2273 X Einthoven et al. v 22204 Holmes 22--73 X Great Britain.

I. SPENCER OVERLHOLSER, Primary Examiner.

MICHAEL V. BRINDISI, Examiner.

20 V. K. RISING, Assistant Examiner.

1. A METHOD OF FORMING COMPOSITE METALLIC OBJECTS COMPRISING HEATING AREFRACTORY MOLD HAVING A CAVITY THEREIN AND HAVING A SOLID METALLICELEMENT POSITIONED THEREIN WHICH FILLS ONLY A PORTION OF SAID CAVITY, TOA TEMPERATURE IN THE RANGE BETWEEN ABOUT 1000*F. AND THE TEMPERATURE OFINCIPIENT MELTING OF THE METALLIC ELEMENT BUT BELOW THE SLUMPTEMPERATURE OF THE MOLD WHILE THE MOLD IS SUBJECTED TO A VACUUM OF LESSTHAN 100 MICRONS, ESTABLISHING A TEMPERATURE LEVEL FOR THE TREATED MOLDAT WHICH THERE IS A TEMPERATURE DIFFERENTIAL IN THE RANGE BETWEEN ABOUT200*F. AND 2000*F. BETWEEN THE TEMPERATURE OF THE TREATED MOLD AND THETEMPERATURE OF METAL TO BE POURED, SAID TEMPERATURE DIFFERENTIAL BEINGOF A MAGNITUDE EFFECTIVE TO PREVENT EXCESSIVE DIFFUSION OF AN ELEMENTBEYOND A BONDING LAYER, POURING THE MOLTEN METAL INTO THE TREATED MOLDTO CONTACT AND TO BOND TO SAID METALLIC ELEMENT WHILE THE METAL AND THEMOLD ARE MAINTAINED UNDER VACUUM AND MAINTAINING VACUUM ON SAID MOLDUNTIL THE CAST METAL IS SOLIDIFIED.